Charge-coupled device with focused ion beam fabrication

Abstract

A charge-coupled device (CCD) is provided with a dopant implant gradient, lateral channel stops and blocking implants by means of a focused ion beam (FIB). The FIB is repeatedly scanned across each cell of the CCD as a succession of overlapping but discrete implant scans. The doping levels of the FIB implants accumulate to a stepwise approximation of a desired dopant density profile, the widths of the steps being no greater than about half the widths of the discrete FIB implants. With a FIB pixel of about 750-1,500 Angstroms, the widths of the steps are preferably about 250-500 Angstroms; the dimension of the cells in the dopant gradient direction can be made less than about 5 microns. The lateral channel stops and back blocking implants can be as narrow as single FIB pixel widths, thus freeing up more of the cell for charge carrying capacity.

We claim: 
     
       1. In a charge-coupled device (CCD) comprising a semiconductor substrate and an array of electrodes having defined electrode lengths and widths for establishing a corresponding array of potential well cells having defined cell lengths and widths in the semiconductor substrate, and for moving charge packets among the cells in the direction of the cell lengths, the cells having respective dopant implant density gradients in the direction of their lengths, the improvement comprising the dopant implants being provided as lateral successions of overlapping but discrete focused ion beam (FIB) implants extending substantially across the cell widths, with each scan displaced from its preceding scan in the direction of the cell lengths, the doping levels of the FIB implants accumulating to a stepwise approximation of a desired dopant density profile having a plurality of steps, the widths of said steps in the direction of the cell lengths being no greater than about half the widths of the discrete FIB implants and no greater than about 750 Angstroms, and the widths of the discrete FIB implants being in the approximate range of 750-1,500 Angstroms. 
     
     
       2. The CCD of claim 1, wherein the widths of said steps are in the approximate range of 250-750 Angstroms. 
     
     
       3. In a charge-coupled device (CCD) comprising a semiconductor substrate and an array of electrodes having defined electrode lengths and widths for establishing a corresponding array of potential well cells having defined cell lengths and widths in the semiconductor substrate, and for moving charge packets among the cells in the direction of the cell lengths, the cells having respective dopant implant density gradients in the direction of their lengths, the improvement comprising the dopant implants being provided as lateral successions of overlapping but discrete focused ion beam (FIB) implants extending substantially across the cell widths, with each scan displaced from and overlapping its preceding scan in the direction of the cell lengths, the doping levels of the overlapping FIB implants accumulating to a stepwise approximation of a desired dopant density profile having a plurality of steps, the widths of said steps in the direction of the cell lengths being no greater than about half the widths of the discrete FIB implants and in the approximate range of 250-750 Angstroms. 
     
     
       4. The CCD of claim 3, wherein the cell lengths are less than about five microns. 
     
     
       5. The CCD of claim 4, wherein the dimension of the cells in the direction of the doping gradient is less than about three microns. 
     
     
       6. The CCD of claim 3, implemented as a buried channel CCD, said cells including lateral channel stops provided as FIB implants having predetermined widths in the direction of their cells widths and extending into the substrate to predetermined depths, the depths of said channel stops being substantially greater than their widths, said channel stops being less than 0.5 microns wide. 
     
     
       7. The CCD of claim 6, wherein said channel stops have substantially single FIB pixel widths in the approximate range of 750-1,500 Angstroms. 
     
     
       8. The CCD of claim 6, said cells including respective blocking implants provided as FIB implants between adjacent cells in the direction of charge packet movement, said blocking implants being positioned to block reverse charge flows from their respective cells. 
     
     
       9. The CCD of claim 3, implemented as a buried channel CCD, said cells including respective blocking implants provided as FIB implants between adjacent cells in the direction of charge packet movement, said blacking implants being positioned to block reverse charge flows from their respective cells, and having predetermined widths in the direction of the cell lengths and extending into the substrate to predetermined depths, the depths of said blocking implants being substantially greater than their widths and their widths being less than 0.5 microns. 
     
     
       10. The CCD of claim 9, wherein said blocking implants have substantially single FIB pixel widths in the approximate range of 750-1,500 Angstroms. 
     
     
       11. In a charge-coupled device (CCD) comprising a semiconductor substrate and an array of electrodes having defined electrode lengths and widths for establishing a corresponding array of potential well cells having defined cell lengths and widths in the semiconductor substrate, and for moving charge packets among the cells in the direction of the cell lengths, said cells including lateral channel stops for blocking lateral charge flows, the improvement comprising the provision of said lateral channel stops as focused ion beam (FIB) channel stop implants having predetermined widths in the direction of the cell widths and extending into the substrate to predetermined depths, the depths of said channel stop implants being substantially greater than their widths, said channel stop implants being less than 0.5 microns wide. 
     
     
       12. The CCD of claim 11, wherein the widths of said channel stop implants are in the approximate range of 750-1,500 Angstroms. 
     
     
       13. In a buried channel charge-coupled device (CCD) comprising a semiconductor substrate and an array of electrodes having defined electrode lengths and widths for establishing a corresponding array of potential well cells having defined cell lengths and widths in the semiconductor substrate, and for moving charge packets among the cells in the direction of the cell length, said cells including reflective blocking implants positioned to block reverse charge flows from their respective cells, the improvement comprising the provision of said blocking implants as focused ion beam (FIB) blocking implants having predetermined widths in the direction of the cell lengths and extending into the substrate to predetermined depths, said blocking implants having substantially vertical side walls aligned with the cell widths, the depths of said blocking implants being substantially greater than their widths. 
     
     
       14. The CCD of claim 13, wherein said blocking implants are less than 0.5 microns wide. 
     
     
       15. The CCD of claim 14, the widths of said blocking implants being in the approximate range of 750-1,500 Angstroms.